Operation of conventional computer equipment, for example, is typically accompanied by the generation of radio frequency and/or electromagnetic radiation in the electronic circuitry of the computer system. If not properly shielded, such radiation can cause considerable interference with entirely unrelated equipment. Accordingly, governmental regulations, as well as sound manufacturing practice dictate that the sources of radio frequency and/or electromagnetic radiation within the computer system be effectively shielded and grounded.
In instances where the radiation-generating circuitry is permanently or semipermanently housed in a container, effective shielding may be accomplished easily through proper construction of the enclosure structure. In certain cases, however, effective RF interference (RFI) or electromagnetic interference (EMI) shielding present somewhat more difficult problems. This is particularly true, for example, where the equipment housing is provided with a readily openable access panel or door, for servicing or other routine access. In addition, large numbers of computer installations have been made in the past without a full understanding of the effect of partially unshielded enclosures, particularly involving access doors. Accordingly, many manufacturers and/or users of older computer equipment are attempting to uprade the level of RFI/EMI shielding through retrofit installation of shielding devices around the access openings. To this end, it has been known heretofore to provide a clip-on shielding device adapted to be pressed or clipped onto an exposed sheet metal edge, typically provided at the peripheral edges of an access door of a conventional sheet metal housing or enclosure for a computer installation. A device of known construction includes a continuous, roll-formed strip metal clip, to one side surface of which has been bonded a resilient tubular element of electrically conductive elastomer. The clip is constructed so as to be appliable over the edge of sheet metal door or panel and retained in place by frictional gripping action of the roll-formed clip. The configuration of the element is such that the resilient tubular elastomer, bonded to the clip, is brought into contact with a fixed surface of the enclosure, when the door or panel is closed, effectively sealing off the narrow gap otherwise typically provided between a door or panel and the oppositely facing surface areas of the housing.
The improved device of my before mentioned prior application Ser. No. 506,010, includes a specially designed continuous metal strip to which is bonded a strip of resilient, conductive elastomer. This device represented a substantial advance in the trade, in providing for high performance shielding with a strip that can be manufactured at reasonable cost and, importantly, which is easily installed, either as part of the initial manufacturing operations or as part of a retrofit installation.
As manufacturers of computer equipment have introduced newer designs, operating at higher and higher frequencies, however, the conductivity of electrically conductive elastomeric tubing, used in the known shielding strips, becomes marginally effective for adequate attenuation of the high frequency spectrum of the radiation. Accordingly, where such higher radiation frequencies are involved, it is becoming more necessary to utilize conductive mesh materials, supported internally by an elastomeric core or similar element, to provide an adequate level of radiation shielding. Radiation shielding strips made for this purpose are known to the trade. However, the manufacturing processes involved in their production are complex and expensive, such that the end product, as installed in the shielded housing, represents a substantial expense.
In the manufacture of conventional mesh shielded strips, for example, a tubular sleeve of conductive wire is knitted in the form of continuous tube closely about the exterior of a resilient core. Typically, this operation is performed twice, once to provide a tightly conforming inner mesh and a second time to provide an outer tubular mesh. Both tubes surround and are supported internally by the resilient core. The thus-shielded core is secured to continuous mounting strip means or the like, for mounting upon the housing to be shielded. Typically, this is done by providing sufficient slack in the outer mesh layer to enable a "tail flange" to be formed of the excess material, which can be secured in some form to mounting means.
While the prior art material described above performs adequately, it is extremely costly. For example, a typical such shielded tubing may sell for as much as 80 cents per foot, for the shielded core alone, without any kind of mounting means.
In accordance with the present invention, a novel and greatly improved construction is provided for mesh shielded resilient core material, which enables this material to be provided to the trade at a small fraction of the cost of conventional material. Pursuant to the invention, a conventional tubular knitted mesh material is formed of a conductive metal wire. The mesh material is not, however, knitted around the core material, but rather is knitted in a conventional manner and gathered in roll form, for example, as flat, double-layered tubular material. Pursuant to the invention, the overall width of the two-layered tubular material is somewhat in excess of the circumference of the resilient core material. This enables the two-layered mesh to be wrapped widthwise around the entire circumference of the core material, leaving the respective opposite side edge margins of the mesh material extending laterally outward from the core. These extending margins are tightly clamped between opposed flanges of a continuous metallic strip, which is advantageously shaped and configured to serve as a mounting clip on the structure to be shielded.
A strip constructed in accordance with the present invention, may be made available to the trade at great cost reduction, without any compromise whatever in the performance of the strip.